node.rs

enum Node {
  Text(String),
  Element(String, Vec<Node>)
}

impl Node {
  fn stringify(&self) -> String {
    match *self {
      Text(s) => s,
      Element(name, nodes) =>
        "<".to_string() + name + ">".to_string() +
        nodes.iter().map(|node| node.stringify()).concat() +
        "</".to_string() + name + ">".to_string()
    }
  }
}

error: type core::iter::Map<'_,&Node,collections::string::String,core::slice::Items<'_,Node>> does not implement any method in scope named concat

nodes.iter().map(|node| node.stringify()).concat()